For the pineapple filling: Puree the pineapple in a food processor and transfer to a medium skillet along with the sugar and 1/4 cup water. Heat over medium-low and bring to a low simmer. Use a rubber spatula to stir and move the mixture around as it reduces and thickens to the consistency of very thick jam, 18 to 20 minutes (you should have about a heaping 1/2 cup). Remove from the heat and stir in the lemon juice. Let cool completely in the skillet. The pineapple filling can be made and refrigerated up to 2 days in advance.
Adjust an oven rack to the middle position and preheat the oven to 350 degrees F. Line a baking sheet with parchment and coat lightly with nonstick spray.
For the macaroons: Whisk the egg whites in a large bowl until frothy. Toss the coconut, sugar, cornstarch, vanilla and 1/4 teaspoon salt together with your hands in another large bowl. Add the coconut mixture to the egg whites and combine with your hands; the mixture should pack together tightly when squeezed. Divide the mixture into 24 slightly heaping tablespoons and roll each into a smooth ball (slightly wet hands can help with rolling). Hold a ball in the palm of your hand and use your thumb to make a slight indentation in the middle. Reshape the edges of the macaroon back into a round if it cracks. Arrange about 1 inch apart on the prepared baking sheet. Bake until golden brown, rotating the tray as needed for even browning, 20 to 24 minutes. Let cool completely on the tray.
Set up a cooling rack over a baking sheet. Combine the chocolate and butter in a small bowl and microwave for 30 seconds; stir. Repeat until all of the chocolate is melted and smooth. Dip the bottom of each macaroon into the chocolate and press into the chocolate about halfway up the macaroon. Use a fork to help lift it out of the chocolate. Transfer to the cooling rack and allow to sit at room temperature until the chocolate hardens and sets up, about 20 minutes.
Fill each macaroon with about 1 teaspoon of the pineapple filling. Store and refrigerate in an airtight container for up to 3 days.
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